The present application relates to unloader valves of air compressors, and is particularly directed to a piston for an unloader valve of an air compressor such as a vehicle air compressor used in an air braking system of a heavy vehicle like a truck.
A truck air braking system includes a vehicle air compressor which builds air pressure for the air braking system. A governor controls system air pressure between a preset maximum and minimum pressure level by monitoring the air pressure in a supply reservoir. When the supply reservoir air pressure becomes greater than that of a preset “cut-out” setting of the governor, the governor controls the compressor to stop the compressor from building air. As the supply reservoir air pressure drops to a preset “cut-in” setting of the governor, the governor returns the compressor back to building air.
The vehicle air compressor is typically a reciprocating air compressor and runs continuously. The compressor runs either in a loaded mode or an unloaded mode. When the compressor is running in the loaded mode, compressed air is delivered to the air braking system. When the compressor is running in the unloaded mode, compressed air is directed to an alternate place which is other than the air braking system. Also, when the compressor is running in the unloaded mode, an unloader valve releases pressurized air building up inside the compressor to reduce the pressurized air in the compressor, which in turn reduces the load on the device driving the compressor. This minimizes power consumption during operation of the compressor in the unloaded mode.
One type of unloader valve includes a balance piston having a stem portion which extends into central opening of a coil spring which, in turn, extends into a larger diameter bore of an unloader piston. The spring may buckle and press against the stem portion of the balance piston. More specifically, air flows back-and-forth between two chambers of the unloader valve as the compressor piston operates back and forth during the loaded mode.
The back-and-forth movement of air flow into and out of the larger diameter bore of the unloader piston takes a finite amount of time to travel up and down the individual coils of the spring, resulting in a pressure differential along the length of the spring. This pressure differential along the length of the spring causes the spring to vibrate. Since the spring may be buckled and pressed against the stem portion of the balance piston, the vibrations of the spring tend to cause premature wear of the stem portion of the balance piston. It would be desirable to provide an unloader valve which overcomes drawbacks of known unloader valves which use a balance piston.